Magnetic tapes have found various applications in audio tapes, videotapes, computer tapes, etc. In particular, in the field of magnetic tapes for data-backup, tapes with memory capacities of several tens to 100 GB per reel are commercialized in association with increased capacities of hard discs for back-up. A backup tape with a capacity of 1 TB or more has been proposed, and it is indispensable for such a backup tape to have a higher recording density.
In the production of a magnetic tape capable of meeting such a demand for higher recording density, advanced techniques are required for production of very fine magnetic powder, highly dense dispersion of such magnetic powder in a coating layer, smoothing of such a coating layer, and formation of a thinner magnetic layer.
To increase the recording density, recording signals with shorter wavelength and tracks with shorter pitches are required, and there has been emerged a system using servo tracks so that a reproduction head can correctly trace the tracks.
To meet mainly the demand for recording of signals with shorter wavelength, magnetic powder for use in magnetic tape have been improved to have more and more fine particle size and also improved in magnetic characteristics. In the field of the existing high recording density magnetic tapes, magnetic powders of ferromagnetic iron oxide, Co-modified ferromagnetic iron oxide, chromium oxide and the like, used in audio systems and household video tapes have been dominantly used. Presently, needle-shape metallic magnetic powder having a particle size in a major axis direction of 100 nm or so has been proposed. On the other hand, to prevent a decrease in output due to demagnetization in recording signals with shorter wavelengths, backup tapes with higher coercive forces have been vigorously developed year by year. As a result of such developments, backup tapes with coercive forces of about 198.9 kA/m have been accomplished by the use of iron-cobalt alloys (JP-A-3-49026, JP-A-5-234064, JP-A-6-25702, JP-A-6-139553, etc.).
In the meantime, the media-producing techniques have been significantly advanced by the development of binder resins having a variety of functional groups, the improvement of the dispersing technique for the above magnetic powder, and further the improvement of the technique of calendering after the coating step. These improvements have markedly improved the surface smoothness of magnetic layers and contributed greatly to an increase in output of signals with shorter wavelengths (for example, JP-B-64-1297, JP-B-7-60504, JP-A-4-19815, etc.).
In association with the recent high density recording, the recording wavelength becomes shorter and shorter. Therefore, in case where the thickness of a magnetic layer is large, the levels of the saturation magnetization and the coercive force of conventional magnetic powder are insufficient within the shortest recording wavelength region, so that the reproducing output decreases to a fraction thereof. Further, because the recording wavelength is very short, self demagnetization loss and thickness loss due to the thickness of a magnetic layer give adverse influences on the resolution, although such demagnetization loss and thickness loss which occur when recorded signals are reproduced have not arisen so serious problem so far. This problem cannot be overcome by the above improvement of the magnetic characteristics of magnetic powder and the improvement of the surfaces of magnetic layers by the medium-producing technique. Under such circumstances, it is proposed that the thickness of a magnetic layer should be reduced.
Generally, it is said that the effective thickness of a magnetic layer is about one third of the shortest recording wavelength used in the system. For example, the thickness of a magnetic layer is required to be about 0.1 μm when the shortest recording wavelength is 0.3 μm. With the trend of compacting a cassette (or a cartridge) for holding tape, a whole of magnetic tape is needed to be thinner so as to increase the recording capacity per volume. To meet such a demand, it is consequently needed to form a thinner magnetic layer. Further, to increase the recording density, a magnetic flux for writing which a magnetic head generates should have a very small area. In this connection, compacting of the magnetic head results in a smaller amount of magnetic flux generated thereby. In order for the above very small magnetic flux to cause a perfect magnetic inversion, it is necessary that a magnetic layer should be formed with a thinner thickness.
However, there arise other problems in the formation of a thinner magnetic layer. That is, when the thickness of a magnetic layer is reduced, the surface roughness of a non-magnetic support gives an adverse influence on the surface of the magnetic layer, so that the surface smoothness of the magnetic layer degrades. When a single magnetic layer is formed with a thin thickness, the solid content in a magnetic paint should be decreased, or the amount of the paint to be applied should be decreased. However, the defects of coating are not eliminated and the filling of magnetic powder is not improved by these methods, which results in poor film strength. To overcome this problem, the following concurrent coating-and-laminating method is proposed: that is, in case where a thinner magnetic layer is formed by an improved medium-producing technique, a primer layer is provided between a non-magnetic support and a magnetic layer, and the upper magnetic layer is applied on the primer layer which is still in a wet state (JP-A-63-187418, JP-A-63-191315, JP-A-5-73883, JP-A-5-217148, JP-A-5-298653, etc.).
When the recording density in the tape-widthwise direction is increased by narrowing the width of the recording tracks, magnetic flux leaking from the magnetic tape is decreased. Therefore, it is needed that MR heads using magneto-resistance elements, which can achieve high output even when the magnetic fluxes are very small, are used for reproducing heads.
Examples of a magnetic recording medium which can correspond to MR heads are disclosed in JP-A-11-238225, JP-A-2000-40217 and JP-A-2000-40218. In the magnetic recording media described in these publications, skewness of outputs from the MR heads is prevented by controlling the magnetic fluxes from the magnetic recording media (a product of a residual magnetic flux density and the thickness of a medium) to a specific value or less, or the thermal asperity of the MR heads is reduced by controlling the dents on the surface of the magnetic layer to a specified value or less.
When the width of the recording tracks is decreased, the reproducing output lowers due to off-track. To avoid such a problem, track servo control is needed. As types of such track servo control, there are an optical servo system (JP-A-11-213384, JP-A-11-339254 and JP-A-2000-293836) and a magnetic servo system. In either of these systems, it is necessary that track servo control is performed on a magnetic tape which is drawn out from a magnetic tape cartridge (or a cassette tape) of single reel type which houses only one reel for winding the magnetic tape, in a box-shaped casing body. The reason for using a single reel type cartridge is that, when the tape-running speed is increased (for example, 2.5 m/second or higher), a tape cannot be reliably run in a two-reel type cartridge which has two reels for drawing out the tape and for winding the same. The two-reel type cartridge has other problems in that the dimensions of the cartridge become larger and that the recording capacity per volume becomes smaller.
As mentioned above, there are two types of track servo systems, i.e., the magnetic servo system and the optical servo system. In the former track servo system, servo track bands, which are explained below, are formed on a magnetic layer by magnetic recording, and servo tracking is performed by magnetically reading such servo track bands. In the latter optical servo type, servo track bands each consisting of an array of pits are formed on a backcoat layer by laser irradiation or the like, and servo tracking is performed by optically reading such servo track bands. Other than these types, there is such magnetic serve system in which magnetic servo signals are recorded on a magnetized backcoat layer (for example, JP-A-11-126327). Further, in other optical servo system, optical servo signals are recorded on a backcoat layer, using a material capable of absorbing light or the like (for example, JP-A-11-126328).
Here, the principle of the track servo will be shortly explained.
As shown in FIG. 7, in the magnetic tape 3 employing the magnetic servo system, the magnetic layer has servo bands 200 for track servo and data-tracks 300 for data recording, each of which extends along the length-wise direction of the tape. Each of the servo bands 200 consists of a plurality of servo signal-recording parts 201 which magnetically record servo-track numbers. A magnetic head array (not shown) for recording and reproducing data onto the magnetic tape consists of a pair of MR heads for servo tracking (for forward traveling and backward traveling), and 8×2 pairs of recording-reproducing heads, where the recording heads are magnetic induction type heads, while the reproducing heads are MR heads. The entire magnetic head array moves in linking with the signals from the MR heads for servo tracking, which read the servo signals. Thereby, the recording-reproducing heads move in the widthwise direction of the tape and reach the data tracks. For example, in the case of the magnetic head array consisting of the 8×2 pairs of the recording-reproducing heads, there are 8 data tracks for one servo track.
However, the improvement of the magnetic powder and the medium-fabricating techniques have now reached the uppermost limit. Particularly in the improvement of the magnetic powder of needle particle type, the particle size thereof in the major axis direction is reduced to about 100 nm as the smallest in view of practical use. This is because, when the particle size is smaller than about 100 nm, the specific surface area of the magnetic powder markedly increases, and the saturation magnetization lowers, and also, it becomes very difficult to disperse such magnetic powder in a binder resin.
The technical innovation of magnetic heads has made it possible to record signals on media having high coercive forces. Particularly in the lengthwise recording system, it is desirable that the coercive force of a magnetic layer should be as high as possible to an extent that the erasing of the recorded signals by a magnetic head is possible, so as to prevent a decrease in output because of demagnetization by recording and reproducing. Therefore, the practical and most effective method for improving the recording density of a magnetic recording medium is to increase the coercive force of a magnetic recording medium.
To suppress the influence of a decrease in output due to demagnetization by recording and reproducing which is the essential problem of the lengthwise recording system, it is effective to further decrease the thickness of a magnetic layer. However, there is a limit in the thickness of a magnetic layer, as long as the above magnetic powder having a needle particle size in the major axis direction of about 100 nm is used. The needle particles are generally arranged such that the needle-pointed direction can be in parallel to the in-plane direction of a medium, because of the lengthwise orientation of the needle particles. However, some of the needle particles are arranged vertically to the plane of the medium, since there is a distribution in the dispersion of the particles. Because of such needle particles, the surface of the medium becomes uneven to increase the level of noises. This problem becomes more serious as the thickness of the magnetic layer is more and more thin.
In case where a magnetic layer is formed with a thinner thickness, it is needed to dilute a paint for magnetic coating with a large amount of an organic solvent. The conventional needle-shape magnetic powder tends to agglomerate paints for magnetic coating. In addition, the large amount of the organic solvent is evaporated off when the magnetic layer is dried, which degrades the orientation of the magnetic powder. Thus, the lengthwise recording tape medium becomes poor in the orientation, and it becomes difficult to obtain desired electromagnetic conversing characteristics therefrom because of degradation of the orientation and the surface of the magnetic layer, even though the magnetic layer is formed thinner. In spite of the known fact that the use of a thinner magnetic layer is effective to improve the recording characteristics in the lengthwise recording system, it is still difficult to obtain a coating type magnetic recording medium which comprises a magnetic layer with a far reduced thickness, as long as the conventional needle-shape magnetic powder is used.
Among several kinds of magnetic powder which hitherto have been proposed, barium ferrite magnetic powder is known which comprises plate particles and has a particle size of about 50 nm (for example, JP-B-60-50323, JP-B-6-18062, etc.). This barium ferrite magnetic powder is more suitable for a thin layer coating type magnetic recording medium, than the needle-shape magnetic powder, because of the particle shape and particle size of the barium ferrite magnetic powder. However, since the barium ferrite magnetic powder is an oxide, its saturation magnetization is about 7.5 μWb/g at most, and therefore, it is theoretically impossible to obtain saturation magnetization of 12.6 μWb/g or more which needle particle type metal or alloy magnetic powder can show. The use of the barium ferrite magnetic powder makes it possible to produce a coating type magnetic recording medium having a thin magnetic layer, but is unsuitable for a high density magnetic recording medium, because the output is low due to low magnetic flux density. Furthermore, the barium ferrite powder particles strongly agglomerate because of the magnetic interaction of the plate particles, and are hardly dissociated to discrete plate particles in a dispersing process. For this reason, the foregoing needle-shape magnetic powder has been dominantly used as the magnetic powder for high density magnetic recording media.
As is understood from the above description, in the formation of a magnetic layer with a thin thickness which is one of the effective methods for improving the recording density of a magnetic recording medium, it is very important to maintain the coercive force and the saturation magnetization of magnetic powder at values as high as possible and simultaneously to reduce the particle size thereof. To achieve this subject matter, the present inventors, firstly, have paid their attentions on the magnetic characteristics of the conventional magnetic powder and found that a theoretical limit is present in achieving a higher coercive force since the conventional needle-shape magnetic powder gains a coercive force based on the shape anisotropy induced by its needle particles. In other words, in the shape anisotropy, the magnitude of the magnetic anisotropy is expressed by 2πIs (wherein ‘Is’ represents saturation magnetization), and is proportional to the saturation magnetization. Therefore, the coercive force of the needle-shape magnetic powder based on the shape anisotropy becomes larger in proportion to an increase in saturation magnetization.
As is well known from the Slater-Pauling curve, the saturation magnetization of a metal or an alloy, for example, a Fe—Co alloy, shows a maximal value at the ratio of Fe/Co of about 70/30. Therefore, the coercive force of this alloy shows a maximal value at the above composition ratio. Needle-shape magnetic powder of Fe—Co alloy in the ratio about 70/30 has already been practically used. However, as has been described above, whenever the needle-shape magnetic powder is used, the coercive force thereof is theoretically limited to about 198.9 kA/m at most at the present, and it is difficult to achieve a higher coercive force under the present circumstances. Therefore, the use of such needle-shape magnetic powder is unsuitable for a thin layer coating type magnetic recording medium.
The magnitude of magnetic anisotropy in the shape anisotropy is expressed by 2πIs as mentioned above, and the coefficient is represented by 2π when the an aspect of magnetic powder (the particle length/the particle diameter) is not smaller than about 5. When the an aspect is smaller than 5, the coefficient rapidly becomes smaller. When the particle shape is spherical, the anisotropy thereof vanishes. In other words, in the state of the art, as long as a magnetic material such as a Fe metal, a Fe—Co alloy or the like is used as magnetic powder, the particle shape of the magnetic powder inevitably and theoretically results in the shape of needle.
As described above, a primer layer with a thickness of about 2.0 μm is formed on an non-magnetic support, and a magnetic layer with a thickness of about 0.15 to about 0.2 μm is formed on the primer layer, in order to improve the characteristics of recording/reproducing of signals with short wavelength. To further improve the recording density, preferably, a magnetic recording medium comprises at least one magnetic layer, and the uppermost magnetic layer (which will simply be referred to as “magnetic layer”) has a thickness of 0.09 μm or less.
In the system of tracking servo signals on a magnetic layer, when the thickness of the upper magnetic layer is reduced, the magnetic servo signals become weak. The solutions for this problem are disclosed in JP-A-04-248120 (see the Examples), JP-A-2000-315312 and JP-A-2002-288817. In these methods, it is described that each of the magnetic recording media has a structure comprising an upper magnetic layer, a non-magnetic intermediate layer, a lower magnetic layer and a non-magnetic support, in which servo signals are also recorded on the lower magnetic layer.
Since these methods use, in the upper magnetic layers, needle-shape magnetic powders with particle sizes of about 100 nm in the major axial direction, some of the needle particles are arranged vertically to the surfaces of the media, as mentioned above. Therefore, the surface smoothness of the media is impaired, and noises are increased because of the roughness of the surfaces of the media. These problems become serious as the thickness of the magnetic layer becomes thinner.
Further, there remains a problem that the use of the needle-shape magnetic powder increases the fluctuation of the thickness of the upper magnetic layer, particularly when the upper magnetic layer is formed with a thickness of as thin as 0.09 μm or less.
Therefore, it is necessary to use magnetic powder comprising substantially spherical or ellipsoidal particles with a particle diameter of 50 nm or less (preferably 30 nm or less) in order to form an upper magnetic layer with a thickness of 0.09 μm or less and to reduce the thickness fluctuation of the upper magnetic layer.
It is preferable to reduce the total thickness of a magnetic recording medium to 6 μm or less by adjusting the total of the thickness of a non-magnetic intermediate layer and the thickness of a lower magnetic layer to 1.4 μm or less, the thickness of a non-magnetic support to 4.0 μm or less, and the thickness of a backcoat layer to 0.5 μm or less. The total thickness of a magnetic recording medium is preferably 5 μm or less, more preferably 4.5 μm or less, still more preferably 4 μm or less. However, it is 2.5 μm or more in view of practical use.
To correctly trace tracks in correspondence with track pitches which have become narrower and narrower, it is necessary that the spacing dimensions between the tape edge and data tracks and between the servo track and data tracks should be kept constant, and thus the higher levels of the dimensional stability against temperature and humidity have been required.